Paula Wallis, TunnelTalk

It is a bitter sweet victory in Toronto, Canada this week as the contractor completes the ill-fated Langstaff Tunnel some 10 months after its original deadline following a massive collapse that trapped the EPBM.

Collapse occurred mid-point on the 3.6km Langstaff Tunnel drive

After a complex recovery operation, inspection of the buried TBM confirmed what engineers had suspected. The standard three-row wire brush seal on the machine was damaged. But was that alone enough to cause the inundation of the tunnel heading, stranding the TBM under more than 1,800m³ of muck? It’s a question that plagues engineers more than 15 months after the collapse of the sewer tunnel for the Regional Municipality of York.

“We still don’t know the actual cause,” said Steve Skelhorn, Project Manager for contractor McNally/Aecon JV. “We think it’s a combination of events. The damaged brushes were probably the catalyst that started the whole sequence of events, but the type of ground we were in is also a factor. The ground came in through the tail brushes causing the machine to sink, the tunnel to sink and ultimately to collapse.”

The questions remain as the contractor completes the much-delayed project. “We’re hoping to be finished this Friday (August 28, 2009) in terms of the permanent work,” said Skelhorn. “We then have a bit of clean up to do, mainly to fix a water main and restore the roadway, but basically the tunnel should be ready for use on Friday.” The contract comprised 3.6km of segmentally lined tunnel, three shafts and ultimately a recovery shaft and surface restoration work to rescue the last drive on the job.

Following the May 2, 2008 collapse the JV went immediately into recovery and mitigation mode to minimize delays in completing the $90 million contract.

Screw conveyor discharge

By August, McNally had launched an available TBM from the other end of the drive to complete the 1.5km back to the recovery shaft. The available TBM was an identical LOVAT EPBM McNally/Aecon had purchased for its 4.1km long 19th Ave sewer contract several kilometers away, also for York Region. That TBM achieved final breakthrough on the 19th Ave job on the same day as the Langstaff tunnel collapse.

The doomed EPBM was excavating east from Keele Street when roughly 1,800m³ of liquefied mud came rushing into the tunnel. Six workers underground at the time escaped unharmed through the access shaft some 1.8km behind the machine. The compromised section of the 3m i.d. tunnel was sealed off with a bulkhead soon after the cave-in.

In December 2008, construction began on the 5m x 30m x 22m deep secant piled recovery shaft over the 100-ton waterlogged TBM buried under more than 20m of soft, saturated, and now disturbed silt and sands.

After digging down to the TBM, the contractor realized part of the machine was lost forever. “When we got down to the TBM and back-up equipment, we found it had sunk below the tunnel alignment by more than 3m, which we did not expect,” said Skelhorn. “As we were restricted by the depth of our secant piles (22m), we took the front portion of the machine out along with the forward shell, the trailing shell and the tail can. We left the screw conveyor, the segment loaders, all the segments and bits and pieces in there and buried them under a mud slab. We couldn’t dig down to them so we really don’t know how far some of it went. It was not visible when we got to the excavation.”

The remains of the $5 million machine were recovered in March 2009 and are evidence in the forensic investigation of the collapse. The tail skin is of particular interest. According to the contractor, it was removed untouched, wrapped up and then uncovered and the brushes inspected in the presence of the client, engineers and the insurance company. Tomislav Hrkac, Project Manager for the owner would not discuss the findings, but said the second TBM launched to finished the drive, was modified. “I can tell you that that machine was fitted with an extra row of brushes to help with any sort of challenging ground conditions,” said Hrkac.

Skelhorn confirmed the extra row of brushes and one other modification. “We put more grease ports on the tail can and automated the system. With the added fourth row of brushes, if we did damage a row we would have a spare one.

Finished Langstaff Tunnel

It just gave us extra insurance.” The modified EPBM headed west toward the cave-in to finished the remaining 1.5km drive within seven months without incident. “It went great,” said Hrkac. “There was on-going dewatering activity at the recovery shaft so the conditions were less challenging, but it went without incident and production rates were great.” “It went extremely well,” said Skelhorn. “We didn’t break any records on it, primarily because we didn’t have a very good shaft to launch the machine. It wasn’t designed for it. We didn’t have a switch in the shaft so we had to remove the whole train bit-by-bit and then put it back down again when we emptied each train. We also didn’t want to arrive at the recovery shaft too early, so working two shifts we averaged about 7m or 8m a shift.”

The second TBM reached the recovery shaft in late February 2009, and was parked for two weeks as crews finished recovering the stranded TBM and installing the mud slab. With both TBMs recovered the tunnel repair commenced, said Hrkac. “There was some jet grouting and some hand mining and other stabilization techniques in the slab area of the recovery shaft and finally the forming and pouring of a tunnel section to create a cast-insitu connection between the segmentally lined drives. The tunnel will be complete this week, with the repair of the water main and repaving of Langstaff Road to be finished by early October.”

Despite the Langstaff collapse, McNally plans to bid on the next major contract in the York Region’s $800 million, 14-segment sewer network known locally as the 'Big Pipe'. The $Can520 million Southeast Collector project

Southeast Collector Tunnel alignment

will employ four LOVAT 3.6m EPBMs, already purchased by the owner, to excavate the 15km tunnel. As the owner prepares to tender the project in January 2010, it confirmed that McNally/Aecon JV, which has completed five major tunneling projects for the utility since 2000, would not be prohibited from bidding on this or future projects.

The contractor would not say how much the TBM recovery and repair of the Langstaff Tunnel would cost, but did say it was unlikely to submit a monetary claim. “It is more likely we will have a claim for additional time for a force majeure,” said Skelhorn. “As a force majeure event, one outside our control, we would be awarded time only, but this would mitigate liquidated damages which are being charged up front and have been imposed since the original finish date of November 5, 2008 was missed.”

At $6,000 a day for every day late, liquidated damages currently stand at close to $1.8 million. “If we were put in the same position again, the same tunnel with what we knew then, the same thing would have happened,” concluded Skelhorn. “There’s nothing we could have been expected to know then, as far as I’m concerned, that would have prevented the collapse. Obviously you can change the world with hindsight, so if we went back into that ground again we would do things differently, but we never expected the unexpected.”

References

Client's EPBM order to jump-start sewer project - TunnelTalk, Aug 2009
Buried EPBM recovery in Toronto - TunnelTalk, Aug 2008